The invention relates to a ballast circuit and method of operating metal halide and xenon-metal halide lamps to reduce or substantially eliminate the detrimental cataphoresis and acoustic resonance effects typically experienced during such metal halide operation.
As disclosed in the cross reference U.S. applications Ser. Nos. 157,359; 157,360; and 157,436, it is desired to provide discharge lamps for automobiles so as to allow the automotive designers to lower the hood lines of the automobile in order to improve the appearance and also the aerodynamic performance of the automobile. As further disclosed in application Ser. No. 157,360, metal halide lamps when operated from an alternating current (A.C.) source such as 60 Hz or from a D.C. source typically experience the effects of cataphoresis which cause the halide of the metal halide lamps to be moved or swept into the end regions of the lamp so as not to contribute to providing the desired illumination of such a lamp. U.S. application Ser. No. 157,360, discloses the use of a vacuum shroud to eliminate the cataphoresis effects associated with the metal halide and xenon-metal halide lamps. While such a vacuum shroud serves its desired purpose, it is desired to eliminate such cataphoresis effects without the need of a vacuum shroud thereby reducing the fabrication costs of such metal halide and xenon-metal halide lamps.
The cataphoresis effects of operating low wattage metal halide lamps are noticeable for a frequency of operation of up to about 1KHz but may be compensated for by supplying excitation to the low wattage metal halide lamps at a relatively high frequency. The high frequency operation, however, may lead to the creation of acoustic resonance effects which degrade the operation of such lamps. Such resonance effects are more fully disclosed in U.S. Pat. No. 4,170,746 of Davenport et al., herein incorporated by reference and to which reference may be made for further details of the detrimental acoustic resonance effects.
As disclosed in U.S. Pat. No. 4,042,856 of Steigerwald in column 4 lines 1-60 the acoustic resonance effects related to gaseous discharge lamps having an applied alternating current (A.C.) may be avoided by a frequency of operation between 10-40KHz associated with the ripple of the A.C. current applied to the lamp.
The cataphoretic transport of metal ions in low wattage gas discharge lamps of the metal halide type may also occur for the operation of such lamps with a direct current (D.C.) excitation. The detrimental cataphoretic transport of metal halide lamps occurring during D.C. operation is particularly noticed when the lamp is oriented in a vertical manner during its operation. The vertical operation provides a gravitational effect drawing or displacing the metal halides from the desired central portion of the lamp and into its end region so as to hinder the desired metal halide ingredients from contributing to the desired illumination of such lamps. It is desired that means be provided to allow for any orientation of the metal halide lamps so as to create a freedom for the automotive designers and lamp fixture designer to position or orient the metal halide lamp in accordance with their particular needs.
Accordingly, it is an object of the present invention to provide for a method of operating gas discharge lamps without the need of a vacuum shroud while at the same time reducing or substantially eliminating the cataphoresis and acoustic resonance effects on the gas discharge lamps and allowing for any desired orientation of these lamps.
It is a further object of the present invention to provide for a ballast circuit that allows for the desired method of operating the gas discharge lamps.
It is a further object of the present invention to provide a method and a ballast circuit for operating various discharge lamps such as a xenon lamp, a metal halide lamp and a xenon-metal halide lamp described in cross referenced related applications have Ser. Nos. 157,359; 157,360; and 157,436 respectively.